In order to obtain oil from oil shale, the oil shale is directly heated by a hot heat carrier (ash) to a temperature of about 500° C. in a rotary kiln. Hereby, oil evaporates from the oil shale forming the so called vapor gas mixture (VGM). The vapor gas mixture (a gas containing also fine particles) is then quenched in a condensation unit for winning the oil. This oil contains particulate material (fines) which traditionally are separated from the oil in a scrubber. The dust particles collected by droplets produced in the scrubber can be found in the cooled oil at the scrubber bottom. The thus dedusted oil is further treated in a rectification column to separate various oil fractions contained in the pyrolysis oil based on their boiling point in a multiple distillation.
Rectification is a standard procedure and described, e.g., in Ullmann's Encyclopedia of Industrial Chemistry, Distillation, chapter 4 Rectification (Multi-stage Distillation), Weinheim 2010, Wiley-VCH Verlag GmbH & Co. KG aA, DOI: 10.1002/14356007.B03—04. pub2. There are, however, several problems in adequately controlling the fractionation separation in the rectification column. Rectification columns usually operate with a substantial amount of reflux reducing the productivity. Further, due to the packings provided in the rectification stages there is a substantial pressure loss over the column.